This is quite literally true. We never know when we might fall ill, only that we will. One day we realize that time has passed us by; that we won't be able to get around to losing those pounds we said we would, that the problem we wanted to solve has gotten away from us. There is no one to blame if that happens. However, we are to blame, and only we, if we do not try our best to achieve all we want to.
Do what you can now. We will get the rest we need and deserve later. That is the one thing we do not need to, and cannot, plan for.